The true power of JavaScript comes not from the language itself but from the DOM, or Document Object Model. This is a set of objects that allow your script to integrate with the browser and work with documents, windows, forms, and other components. This lesson introduces the DOM and shows how to use its objects to work with documents and browser history.

When I first wrote about JavaScript in 1996, I was excited about its potential for the Web. While it had its limitations, it went beyond simple HTML and offered true interactivity to Web pages. In the years since then, JavaScript has lost most of its limitations and has become an essential part of the Web. Image rollovers, pop-up messages, and form validation are now commonplace, and JavaScript is still the language of choice for these and many other applications.

My book, "Sams Teach Yourself JavaScript in 24 Hours," is now in its third edition, revised and updated to cover the latest browsers and features. This book starts with the fundamentals of JavaScript—statements, variables, functions, strings, and objects—and moves on to a wide variety of complete examples, including sophisticated techniques such as Dynamic HTML.

While JavaScript is widely used on today's Web, it is by no means widely understood—a great many pages are created by copying scripts from other pages, or from books. While this book certainly has some great scripts to copy, it also encourages you to step beyond simple cut-and-paste scripting and really get a feel for scripting from scratch. Even if you've never written anything but HTML so far, you'll find it easy to learn JavaScript, and you'll be creating interactive pages with scripts in no time.

This chapter is from the book

This chapter is from the book

You've arrived at Part III of this book. (If you've been reading
nonstop, it's been eight hours, so you might want to get some sleep before
you continue.) In this part, you'll explore some of the most important
objects used with JavaScript.

In Hour 8, "Using Math and Date Functions," you learned about JavaScript's
support for objects, which allow you to store data in all sorts of interesting
ways. However, the objects you'll use the most are those in the Document
Object Model (DOM), which let your scripts manipulate Web pages, windows, and
documents.

In this hour, you will explore this hierarchy of objects. Hour 9 covers the
following topics:

How to access the various objects in the DOM

Working with windows using the window object

Working with Web documents with the document object

Using objects for links and anchors

Using the location object to work with URLs

Getting information about the browser with the navigator object

Creating JavaScript-based Back and Forward buttons

Understanding the Document Object Model

One advantage that JavaScript has over basic HTML is that scripts can
manipulate the Web document and its contents. Your script can load a new page
into the browser, work with parts of the browser window and document, open new
windows, and even modify text within the page dynamically.

To work with the browser and documents, JavaScript uses a hierarchy of parent
and child objects called the Document Object Model, or DOM. These objects are
organized into a tree-like structure, and represent all of the content and
components of a Web document.

Like other objects you've explored, the objects in the DOM have
properties, which describe the Web page or document, and methods,
which allow you to work with parts of the Web page.

When you refer to an object, you use the parent object name followed by the
child object name or names, separated by periods. For example, JavaScript stores
objects to represent images in a document as children of the document
object. The following refers to the image9 object, a child of the
document object, which is a child of the window object:

window.document.image9

The window object is the parent object for all the objects we will
be looking at in this hour. Figure 9.1
shows this section of the DOM object hierarchy and a variety of its objects.

This diagram only includes the basic browser objects that will be covered in
this hour. These are actually a small part of the Document Object Model, which
you'll learn more about starting with Hour 18, "Working with Style
Sheets."

History of the DOM

Starting with the introduction of JavaScript 1.0 in Netscape 2.0, JavaScript
has included objects to represent parts of a Web document and other browser
features. However, there was never a true standard. While both Netscape and
Microsoft Internet Explorer included many of the same objects, there was no
guarantee that the same objects would work the same way in both browsers, let
alone in less common browsers.

The bad news is that there are still differences between the
browsersbut here's the good news. Since the release of Netscape 3.0
and Internet Explorer 4.0, all the basic objects (those covered in this hour)
are supported in much the same way in both browsers, and new DOM standards are
supported by the latest versions of Netscape and Internet Explorer.

While all this standardization doesn't change how the objects described
in this hour work, you'll be thankful for it as you move into the advanced
features of the DOM later in this book.

DOM Levels

The W3C (World-Wide Web Consortium) has recently developed the DOM level 1
standard. This standard defines not only basic objects, but an entire set of
objects that encompass all parts of an HTML document. A level 2 DOM standard is
also under development.

The basic object hierarchy described in this hour is informally referred to
as DOM level 0, and the objects are included in the DOM level 1 standard.
You'll learn how to use the full set of Level 1 DOM objects in Part V of
this book.

TIP

The Level 1 and Level 2 DOM objects allow you to modify a Web page in real
time after it has loaded. This is called dynamic HTML (DHTML) and you'll
learn more about it in Part V.